<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    .box{
        height: 100px;
        width: 100px;
        background-color: red;
        outline: 10px solid #0f0;
    }
    /* 行内级元素设置margin-top无效 */
    a{
        margin-top: 20px;
        display: inline-block;
    }
</style>
<body>
    <div class="box"></div>
    <a href="">百度一下</a>
</body>
</html>